The invention relates to an electronic device for a cable broadcasting system provided on a transmission line from a center equipment to a terminal device for processing broadcasting signals transmitted on the transmission line, and more specifically to an electronic device for a cable broadcasting system provided with a microcomputer as a control circuit.
Heretofore, a tap device, an amplifier, a distributor and the like are known as electronic devices for a cable broadcasting system such as a CATV system provided on a transmission line made of a coaxial cable and the like from a center equipment to a terminal device, for processing broadcasting signals such as signals of television signals and the like. Some of such electronic devices are known to be designed to set the operation mode in response to command signals from the center equipment.
A tap device will be described as an example of an electronic device that can set the operation mode, as follows.
Such a tap device is provided with a directional coupler for branching a part of broadcasting signals passing through the transmission line and outputting the branched broadcasting signals from tap output terminals to terminal devices, a latching relay (simply called a xe2x80x9crelayxe2x80x9d, hereinafter) provided in a signal pass transmitting the branched broadcasting signals to the tap output terminal for changing over the condition of on or off of the signal pass, an operation circuit for changing over the condition of on or off of the signal pass by operating the relay, a receiving circuit for receiving command signals from the center equipment, a control circuit comprising a microcomputer for setting the operation mode (in the case, the condition of on or off of the relay) and a power source circuit for receiving power signals from a power supply device via the transmission line and supplying the receiving circuit, control circuit and operation circuit with electricity.
When such a tap device is provided on a transmission line, a individual address is given to the tap device in order to discriminate it from other electronic devices on the transmission line. When the center equipment changes over the output of the broadcasting signals from the tap output terminal of the tap device, it produces a command signal made by giving an address characteristic to the tap device to a command data (simply called a xe2x80x9ccommandxe2x80x9d, hereinafter) indicating content of a command, and transmits the command signal to the transmission line after converting it to a transmission signal. Then, in the tap device, the receiving circuit receives only the transmission signal, reconverts the signal to a command signal and outputs it to the control circuit. After receiving the command signal, the control circuit extracts the command signal from the center equipment by selecting a command signal with the same address among various comand signals, and outputs or stops broadcasting signals from the tap output terminal by operating the relay in response to the command contained in the command signal.
Such an electronic device capable of setting the operation mode in response to a command from the center equipment is provided with a control circuit including a microcomputer, in order to set operation mode in response to a command signal from the center equipment.
In such an electronic device, whose control circuit comprised a microcomputer, command signals from the center equipment occasionally could not be received because of misoperation of the microcomputer due to noises induced by, for example, a thunderbolt. In the case, since command signals could not be outputted to the microcomputer, the microcomputer had to be reset to be operated from the initial state. Heretofore, the operator had to be specially dispatched to the site of the tap device to push the resetting switch provided in the electronic device, in order to reset the microcomputer. It was very troublesome to do such works. That is, hitherto, the works for recovering normal state of the microcomputer had to be done manually.
Especially when the electronic device was a tap device, there happened problems such that broadcasting signals could not be transmitted for long time to the subscriber""s terminal where the broadcasting signals should have been outputted from the tap output terminal of the tap device, that there came a telephone call of complaint from the subscriber that the television did not work or that there happened a loss of transmitting broadcasting signals to a subscriber""s terminal where the broadcasting signals should not have been transmitted without fees for long time until the system w-as recovered, because it took long time until the operator was dispatched after the trouble was discovered by the center equipment.
An object of the invention is to provide an electronic device which can be automatically recovered to its initial state by resetting its microcomputer when it becomes impossible for the electronic device to receive command signals because of misoperation of the microcomputer due to a thunderbolt and the like, in a cable broadcasting system in which operation mode of the electronic device provided on a transmission line is designed to be set by the center equipment by transmitting a command signal from the center equipment to the transmission line.
In an electronic device of the invention, a microcomputer counts an input stop period of a command signal from a receiving circuit and stops output of a pulse signal to a watchdog timer when the input stop period gets to a predetermined time longer than a time interval at which a center equipment outputs command signals periodically.
On the other hand, the watchdog timer counts an output stop period of a pulse signal for monitoring operation condition outputted periodically from the microcomputer. When the counted output stop period gets to a predetermined abnormality determining time, the watchdog timer outputs a reset signal to the microcomputer to restart it from the initial state.
Therefore, according to the invention, even when the microcomputer becomes unable to receive command signals owing to noises inputted to the control circuit by, for example, a thunderbolt, command signals can be inputted to the microcomputer and the electronic device can automatically regain its normal function, by restarting the microcomputer and setting its initial state.
Then, in an electronic device of another aspect of the invention, the microcomputer counts an input stop period of the command signals to the electronic device itself as an input stop period of the command signals from the receiving circuit. When the counted input stop period gets to the predetermined time, output of a pulse signal to the watch dog timer is stopped. Specifically, operation signals for the electronic device itself as well as for other devices are inputted to the receiving circuit as far as the signals are command signals from the center equipment. In addition, the electronic device of the invention extracts only the signals for the device itself, counts the input stop period and determines whether the pulse signal should be outputted or stopped by comparing the period with a predetermined time interval at which signals are outputted periodically from the center equipment.
Therefore, according to the invention, when broadcasting signals are not transmitted to the subscriber""s television set due to various troubles like thunderbolt, a reset signal is outputted to the microcomputer and it is restarted from the initial state, as mentioned above.
According to an electronic device of a further aspect of the invention, the microcomputer changes the predetermined time in response to a change command when a command signal received by the receiving circuit is a command for changing the predetermined time for the electronic device itself.
Therefore, according to the invention, the electronic device selects only the command signals to the device itself among all command signals received by the receiving circuit and can change the predetermined time in response to the change command when the command signal is the command for changing the predetermined time.
Hence, according to the invention, the electronic device can be easily applied to a cable broadcasting system in which transmission intervals of command signals from the center equipment differ for each of the command signals, thereby electronic devices with a wide use being provided.
In the furthest aspect of the invention, any of the electronic devices of the invention described above is applied to the tap device as an electronic device which sets operation mode in response to a command signal from the center equipment.
The tap device is provided with a directional coupler for branching a part of broadcasting signals and outputting the branched broadcasting signals from the tap output terminal. In the signal pass from the directional coupler to the tap output terminal, a change-over device such as a relay and the like for changing over the condition of on or off of the signal pass is provided. The tap device is also provided with an operation circuit for changing over the condition of on or off of the signal pass by operating the change-over device. The control circuit sets the condition of on or off of the signal pass in response to a command signal from the receiving circuit. The operation circuit changes over the condition of on or off of the signal pass by supplying the change-over device with electricity under the control of the control circuit. A power source circuit receives power signals from the power supply device via the transmission line and supplies the receiving circuit, control circuit and operation circuit with electricity.
Therefore, according to the tap device of the furthest aspect of the invention, when a command signal received by the receiving circuit is a command for the tap device itself for changing over the signal pass, the microcomputer operates the relay via the operation circuit in response to the command and changes over the condition of on or off of the signal pass by outputting a command for changing over the signal pass as a command signal from the center equipment.